DSA interview questions and answers

Track the farthest reachable index while scanning left to right

If total gas is less than total cost, no start works.

Sort by end time and keep an interval if it starts after or at the last kept end

Sort items by value/weight and take whole items then a fraction of the next, which is optimal for the fractional version.

Slide a min-heap of size k+1: push the next element and pop the min into the result.

There are n!

A geometric resize (double the capacity) copies O(n) elements only occasionally.

Backtrack: at each index either include nums[i] or skip it, recording a copy when i reaches n.

Backtrack by swapping the next position with each remaining candidate, or by choosing unused indices with a used array.

XOR everything

I scan once and store seen values in a hash set or map.

I push opening brackets and pop when a matching closer arrives.

Ordering tasks with prerequisites in a DAG.

Big-O upper-bounds how time or space grow as input size grows, ignoring constants.

Preorder with null markers or level order with queue.

Scan tokens, push numbers, pop operands for operators.

BFS for shortest path in unweighted graphs and level order.

Overlapping subproblems plus optimal substructure.

When you need ongoing top-k or priority extraction.